1. Field of the Invention
This invention relates to the manufacture of clad composites--for example, copper clad composite for use as printed circuit wiring board. A batch-type process is commonly used in the manufacture of such composites. In this application a method is disclosed by which these clad composites can be manufactured in a continuous, single step process. It is estimated that this new technique can reduce manufacturing costs by as much as fifty percent.
2. Description of the Prior Art
The manufacture of resin-impregnated composite bodies has gone through a period of considerable technical evolution during the past half century. At the start of this period such composites were formed by means of wet lay-up techniques. In these procedures an assembly of fibrous material is impregnated with an appropriate resin which is subsequently cured. Two different wet lay-up techniques were prevalent. In the hand lay-up technique successive layers of fiber and liquid resin are built up. In the spray lay-up technique a mixture of fiber and resin is sprayed on an appropriate base to a predetermined thickness. Depending on the use envisioned for the final product, cladding might be situated on the resin-wet fiber. The resin is then cured in order to fully cross-link the polymeric structure. The result is a hard clad composite such as a metal clad composite, which might be used as printed circuit wiring board. The principal drawbacks in these techniques is that they require large amounts of hand labor, the quality of the final product is largely operator dependent and they are batch-type processes which do not lend themselves to continuous manufacturing, with the cost reduction that naturally flows from such continuous techniques.
A considerable improvement in composite manufacturing occurred when it was found that resin impregnation and partial curing could be effected in a single continuous procedure. In this procedure continuous fibrous reinforcement material, either woven or unwoven, is unwound from a large spool, immersed in a resin bath and drawn passed metering devices to control resin deposition. The resin-impregnated fiber reinforcement is then guided through an oven where the resin is only partially cured and dried. This partially cured composite is then formed into large rolls, perhaps with interleaved polyethylene to prevent sticking, for later use. The fibrous reinforcement which is resin impregnated and partially cured is referred to as "prepreg." In the production of clad composite the previously supplied prepreg is cut into large sheets, overleaved with cladding and heated under pressure. During this heating the resin is fully cured and the cladding is thusly attached to the composite. One sheet of prepreg may be used, or a number may be stacked to form a laminate. The continuous manufacture of the prepreg was made possible only through the careful control of resin content, the development of resins, catalysts, and other additives capable of intermediate curing with only partial crosslinking, and the highly controlled deposition of resin on the reinforcement material.
While the prepreg technique is the current state of the art process for the production of clad reinforced composite, the technique is still not amenable to continuous production. The final pressing and curing of the composite and the cladding is a batch step, and as such much of the advantage gained in the continuous production of the prepreg is not fully realized. The instant invention however, provides for the continuous single-step production of clad composites.
A collateral development which adds to the meaningful light in which the instant invention might be viewed is the introduction of molded plastics, specifically pultruded composites. There exist many techniques for molding plastics including, for example, liquid die injection and the use of split molds. However, as is the case for most of the molding techniques, these are batch-type processes and, consequently, do not exhibit the economic advantages inherent in continuous production methods. A more recent development allows for continuous production of molded plastics. In this process--referred to as pultrusion--a fibrous material is impregnated with resin and then pulled through a die. Significant heating and compression take place as the material is "pultruded" through the die, and the result is a fully cured and totally cross-linked reinforced plastic. Depending on the shape of the die, the completed product assumes any spatial characteristics desired and practical. In this process there is no intermediate product which is partially cured, as in the prepreg process. This is rather a single-step technique in which the resin is taken from the noncured "A stage" to the fully cured "C stage", and is molded, in one single continuous process. Although there is significant compression, and consequently great pressure, as the large amount of resin-impregnated fiber is forced into the die and pulled through it, the viscosity of the resin provides some natural lubrication and additional chemical components may be added to alleviate any adhesion to the die walls. In order to minimize such adhesion at the exit end of the die, where the resin is essentially fully cured and dry, only those resins which contract on curing are used.